The present invention relates to a method of gasifying and agitating a micro-organism culture which enables its growth to be accelerated. It also relates to a fermenting apparatus for putting this method into practice.
It is known that there are at the present time two chief methods for homogenising a culture medium and for increasing the rate of dissolution of air with a view to growing aerobic micro-organisms (cells, fungi, bacteria, etc). One of these is mechanical and consists in agitating the medium mechanically while injecting into it the sterile air needed for the development of the micro-organisms, while the other referred to as the "airlift" method, consists in imparting motion to the medium directly using bubbles of air which are fed continuously at the base of the fermenter.
The first method has a number of disadvantages. Firstly it calls for fermenters whose structure is far more complicated since they include a mechanical assembly with a drive motor, agitator members, sealing glands, etc. In addition such fermenters are ill suited to continuous operation because of the sterility problems which arise as a result of the structural complexity of the fermenter which must include rotary joints and so on.
The airlift method referred to above can be put into effect with far simpler apparatus which is able to operate continuously and which does not raise the sterility problems which exist with mechanical methods. However, a major disadvantage of methods of this kind lies in the consumption of air which they involve, which is very much greater than with mechanical methods. To give an example, at a comparable rate of growth, in cases where mechanical methods would call for a consumption of 1 v.v.m (volume of air per volume of culture per minute), the airlift method calls for a consumption which may be of the order of 3 v.v.m. If this consumption is reduced, the medium is not adequately agitated and the rate of growth of the micro-organisms falls off sharply. However, the air which is admitted into a fermenter is an expensive commodity since it must have been very thoroughly sterilized beforehand to prevent or minimise any contamination of the medium. Thus, in airlift methods the cost of a culture of given density or microbial concentration depends in essence on the initial seeding cost, on the cost of the nutrient medium, and on the cost of the energy consumed and it is this latter, which is proportional to the consumption of sterile air, which is generally the dominant factor.
It is an object of the invention considerably to reduce the importance of the factor mentioned above and to this end it aims to provide an improved method of gasifying and agitating a micro-organism culture which also benefits from the advantages of the airlift method.
In other words, an object of the invention is, in any given application, either to increase the density of micro-organisms achieved for a given air consumption, or to reduce considerably the consumption of air required to obtain a given density, or again to have a favourable effect on both these factors in order to allow micro-organisms to be produced under the optimum conditions in any set of practical circumstances.
In a similar fashion to the airlift method, the method to which the invention relates calls for the culture to be placed in a fermenter comprising at least two containers which communicate with one another in their upper and lower regions. The method according to the invention consists in injecting substantially sterile air in the form of bubbles discontinuously at the base of one of the containers, with injection periods of predetermined length and non-injection periods alternating at a predetermined frequency, thus having a surging or pulsating effect on the culture in the two containers, a general flow between the containers taking place through the intercommunications between them.
As will better be appreciated at a later stage, this surging or pulsating effect on the culture is produced by the alternating infeed of air and considerably increases the movement in the culture and its growth. For example, with an identical mean consumption of the substantially sterile air, it is found that the method of the invention on the one hand enables the length of the latency phase i.e. the initial phase in the course of which the rate of growth is very slow to be reduced in comparison with the conventional airlift method, and on the other hand enables the rate of growth during development to be very considerably increased, with the result that, at the end of a given period of operating the method, the micro-organism density in the culture is considerably greater than the density which would have been achieved under the same operating conditions with the conventional method, while the consumption of air will have been the same. By way of illustration, the ratio between the densities may be of the order of 3:1.
In practice, the frequency of the injections of air will be equal to at least one per minute, with the aim of producing an effective surge effect. Furthermore, experiments have shown that good results can be obtained with non-injection periods whose length is of the order of 0.5 to 10 times that of the injection periods.
In a first manner of putting the method into practice, the injection frequency and the length of the injection periods are constant and are present to suit the oxygen requirements of the micro-organisms and the degree of agitation required. The settings may be arrived at by conducting preliminary tests with the strain of micro-organism concerned.
In another manner of implementation, the frequency of injection and the length of the injection periods are variable and are controlled at all times by sensors which are associated with the culture to allow its state to be detected, with a view to meeting the requirements of the organisms in the optimum fashion at each stage of growth.
The invention also relates to fermentation apparatus for putting into effect the method described above. This apparatus is of the airlift kind and comprises two containers which communicate with one another in their upper and lower regions, and an air injector which opens into one of them: the latter may be formed by a vertical column arranged in the other container, which latter is itself formed by a vertical column of larger diameter. The air injector is advantageously associated with sequential air supply means which are adapted to supply substantially sterile air for periods of predetermined length at a predetermined frequency.
The two containers preferably communicate in their lower regions via a venturi member which contains a constriction, the air injector opening into the first mentioned container close to this constriction in order, during the injection periods, to produce an effect whereby the culture is drawn from the other container into the first container. The surge effect is thereby considerably increased.